1. Field of the Invention
The present invention generally relates to the art of applying a liquid coating material to a web of film such as, for example, a photographic film or a magnetic recording tape. More specifically, the present invention relates to a coating head assembly for, and a method of, applying a liquid coating material to a web of film to form thin parallel strips of coating material on the film web.
2. Description of the Prior Art
When it comes to the application of a coating material on a web of film such as, for example, a web of photographic film or a web of magnetic recording tape, various method have been well know including, for example, a gravure roll coating technique, a reverse roll coating technique and so on. These methods have their own applications and the selection of one of these methods depends on the purpose for which it is utilized. Of these various method, the reverse roll coating system is employed where parallel strips of coating material extending over the length of the web of film and spaced equally over the width thereof are desired to be formed on the film web. Since the present invention pertains to the formation of the parallel strips of coating material on the film web, the prior art reverse roll coating system will now be discused in detail with particular reference to FIG. 5.
Referring to FIG. 5, the prior art coating machine comprises a rotatably supported back-up roll 72 around which a web 71 of film is turned during its transportation in one direction, a transfer roll 73 juxtaposed with the back-up roll 72 for rotation at a predetermined speed in one direction counter to the direction of rotation of the back-up roll 72 and having a plurality of axially equally spaced annular grooves defined therein in a circumferential direction thereof while leaving a corresponding number of annular lands on the peripheral surface of such transfer roll, and an applicator roll 74 juxtaposed with the transfer roll 73 for rotation in a direction counter to the direction of rotation of the transfer roll 73 and poitioned on one side of the transfer roll 73 remote from the back-up roll 72. The machine shown therein also comprises a paint reservoir 76 in which the applicator roll 74 is partly immersed so that, during the rotation of the applicator roll 74, a coating material within the paint reservoir 76 can be applied onto and transferred by the applicator roll 74 towards the transfer roll 73, a metering roll 75 for metering the coating material being transported by the applicator roll 74 towards the transfer roll 73, and a squeegee blade 77 for removing the coating material sticking on the peripheral surface of the metering roll 75 for the recovery thereof into the paint reservoir 76.
The prior art coating machine of the above described construction operates in the following manner. Assuming that the coating machine is electrically powered with the applicator roll 74 driven at a predetermined peripheral velocity, the applictor roll 74 is successively immersed in the paint reservoir 76 so that the peripheral surface of the applicator roll 74 can be wetted with the coating material within the paint reservoir 76. The coating material transported by the application roll 74 is subsequently metered by the metering roll 74 and is then transferred onto the transfer roll 73, adhering to the annular lands on the peripheral surface of the transfer roll 73 so that portions of the coating material transferred onto the annular lands can subsequently be applied to the film web 71. The application of the coating material from the transfer roll 73 to the film 71 takes place while the film web 71 is continuously transported in the predetermined direction around the back-up roll 72 and, therefore, striped deposits of the coating material can be formed on one surface of the film web 71.
In the prior art reverse roll coating machine of the type referred to above, the maximum coating speed is said to be generally to about 200 m/min, for there may be a possibility that at a higher coating speed the coating material carried by any one of the various rolls may scatter under the influence of a centrifugal force induced by the rotation of the associated roll.
Also, since the coating material, liquid in phase, contacts the ambient air, the coating material if diluted with a solvent prior to the actual application thereof tends to evaporate in contact with ambient air enough to facilitate change in its characteristics with time. Therefore, the deposits of the coating material tend to become susceptible to deterioration. Considering that rationalization of the coating process is required to facilitate a mass-production of coated products, the above described problems are not negligible.
On the other hand, where the coating material diluted with a solvent is employed, the coating material deposited on a film web undergoes a considerable volume shrinkage as the deposited coating material is cured or hardened in a drying process. The magnitude of the volume shrinkage varies with the type of one or both of the coating material and the solvent and will generally amount to about 1/5 to 1/20 relative to the thickness of the coating material as coated on the film web. The shrinkage and the hardening occurring at this time will now be discussed. The coating material on the surface of the film web is held immovable, having satisfied a NO-Slip condition referred to in the field of fluid dynamics, and therefore, no apparent change is observable at the interface between the coating material and the film web while a free fluid above the interface undergoes a motion consequent upon the shrinkage to thereby determine a final pattern or shape of distribution of the coated material. A diagrammatic representation of the coating material in motion above the interface between it and the film web is illustrated in FIG. 6, wherein reference numeral 1 represents a portion of the film web, reference numeral 2 represents an uncured coating material applied to the film web portion 1 and reference numeral 3 represents the coating material having been dried to form a coated layer. As can be understood from FIG. 6, the coated layer 3 has its edges rounded as indicated by circles A.
When it comes to the continuous formation of strips of coated layers spaced an equal distance from each other over the width of the film web, variation of the uniformity in thickness of the coated layers resulting from run or sagging of coating material at edges thereof may pose a critical problem because the neighboring coated layers are spaced a small distance from each other.